Battery blocks, each composed of a plurality of unit cells accommodated in a battery case in such a way as to allow a predetermined current to flow at a predetermined voltage, are widely used as power sources for various applications including various devices and vehicles. A recent trend is the use of technology in which battery blocks are combined into a module and different combinations of modules are used depending on different applications. Furthermore, an improvement in the performance of the unit cell reduces the module's size and weight, as well as improves the working efficiency in the assembling of a battery block. The technology of combining modules offers advantages such as improved flexibility in arranging a power source in a limited space of, for example, a vehicle.
As secondary cells are improved in their performance, it is increasing important to attain the safety of the module in addition to the safety of the unit cell itself. In particular, when the heat of abnormal heat generation due to an internal short circuit or other events in one unit cell is transferred to another unit cell in the module, a normal unit cell also undergoes degradation of characteristics and may cause abnormal heat generation. As a result, this may further trigger a chain reaction of degradations and abnormalities of the accommodated unit cells.
To overcome the foregoing problem, PTL 1 proposes providing a battery case, which is composed of thermally conductive cylinders for accommodating therein secondary cells, with plastic walls formed integrally with the cylinders for preventing thermal runaway. With this method, the walls prevent the radiant heat from transmitting from an abnormally heated secondary cell to nearby secondary cells.
FIG. 17 is a conceptual diagram showing heat transfer in a conventional battery block described in PTL 1.
In FIG. 17, reference signs 1A and 1B each denote a secondary cell. Reference sign 3 denotes a wall made of plastic for preventing thermal runaway. The clearances between the surface of wall 3 and the surfaces of secondary cells 1A and 1B are 0.5 mm or less. The surfaces of secondary cells 1A and 1B are in contact with the surface of wall 3. The thermal conductivity of the plastic forming wall 3 is between 0.05 and 3 W/(m·K) inclusive. The thickness of wall 3 is between 0.5 and mm inclusive. In the battery block, radiation heat from secondary cell 1A to secondary cell 1B is blocked by wall 3. Wall 3 releases the heat generated in secondary cell 1A to secondary cell 1B by conduction. This prevents thermal runaway in neighboring secondary cells.
Battery blocks with improved absorption of heat from unit cells to the battery case have also been known. Examples thereof include battery blocks where the thickness of rows of walls positioned at the central portion of the battery case is made larger than the thickness of those positioned at the peripheral portion (e.g., see PTL 2); battery blocks in which a relationship between the diameter of unit cells accommodated in the battery case and the distance between two neighboring unit cells is specified (e.g., see PTLs 3 and 4); and battery blocks configured to absorb heat generated in unit cells by means of a battery case having a large thermal capacity (e.g., see PTL 5).
Moreover, battery blocks with improved heat releasing from the battery case have been known. Examples thereof include battery blocks having on the outer surface of the battery case a heat radiation layer having a heat radiation capability higher than that of the battery case (e.g., see PTL 6); battery blocks having a thermally conductive layer between the battery case and unit cells (e.g., see PTLs 7 and 8); and battery blocks where the thickness of the battery case is relatively reduced at the central portion for increased heat radiation capability at the central portion (e.g., see PTL 9).